After I bought 3 cobra guppies yesterday, at first they were fine, now I saw a tail of one of them is missing a piec at the tip, looks likes its nipped probably by my betta! Also my betta use to be fine with my old guppy now I saw one of my old guppy tail is shred too! Not way too bad but still bad. How long the tail of my guppies grow back? Also plz don't suggest separating them because I can't right now. Is my betta just in a bad mood? Or wat?????

I'm sorry, but the only thing we can suggest is separating them. They are not children, you can't tell him to stop terrorizing the others. They will do whatever they want. Out of curiosity, how big is your tank?

And whenever adding new tankmates (especially with bettas) you need to have a plan B. Either take the betta back to the store, take the other fish back, or out the betta in another tank or get a divider as a temporary solution. This is the ONLY way to stop it. Your betta will most likely kill the other fish if he has started attacking them.

In the mean time, the injured fish need salt treatments to heal. More info on your tank will help with the doseages needed.

Your guppy are not good with betta because the male guppies have very big tail which slow them down alot and that makes it a easy target for the betta they can be aggressive towards slower fish. If you can't find a another tank for your guppies best thing to do is put your betta into a smaller bowl until you can get the betta a bigger bowl.